There is increasing need for higher data rates, better efficiency, and support for larger numbers of users in the sophisticated wireless data communications devices that are deployed worldwide. For example, the IEEE 802.11 Wireless Local Area Network (WLAN) systems that have achieved widespread use utilize Multiple Input Multiple Output (MIMO) techniques to increase communication data rates and hence support greater data traffic to larger numbers of users. Further advances in WLAN systems utilize Multi-User MIMO (MU-MIMO) techniques to allow concurrent transmissions to be made in the downstream direction from IEEE 802.11 Access Points (APs) to client devices. This significantly improves the efficiency of the system and the utilization of the wireless channel.
FIG. 1 illustratively represents an MU-MIMO data transfer process as may be implemented in an MU-MIMO IEEE 802.11 protocol such as IEEE 802.11ac. For descriptive purposes FIG. 1 depicts a possible downstream MU-MIMO data transfer by the AP followed by upstream data transfers by the clients. The downstream MU-MIMO frames (Physical layer Protocol Data Units, or PPDUs) simultaneously transmitted by the AP in one block are 1, 2, 3, 4, with padding 5 being used to pad out the PPDUs to the same transmission duration. PPDUs 1, 2, 3, 4 are transmitted to four different clients at the same time. After the AP transmits these PPDUs, the corresponding clients return Acknowledgements 6, 7, 8 and 9 respectively, waiting a Short Interframe Space (SIFS) from the AP's PPDUs. The clients now transmit data upstream to the AP as their own PPDUs 10, 12, 14, 16, waiting the appropriate Distributed Coordination Function Interframe Spacing (DIFS) and SIFS intervals, and receiving Acknowledgement frames 11, 13, 15, 17, as dictated by the IEEE 802.11 protocol. As MU-MIMO does not support concurrent upstream transmissions, the clients are forced to send their PPDUs 10, 12, 14, 16 separately.
It is evident from FIG. 1 that while substantial efficiency is possible in the downstream direction from the AP to clients by virtue of MU-MIMO, the upstream direction does not enjoy such efficiencies. Current technology improvements in IEEE 802.11 are adopting advanced techniques such as Orthogonal Frequency Division Multiple Access (OFDMA) to permit concurrent transmissions to also be made in the upstream direction from IEEE 802.11 client devices to APs. This further improves the ability of a WLAN system to support many concurrently active client devices on each AP.
FIG. 2 represents for descriptive purposes an MU-MIMO exchange employing OFDMA in the upstream direction. As in the preceding case, the AP utilizes MU-MIMO techniques to transmit PPDUs 1, 2, 3, 4, padded with padding 5 (and preceded by Physical Layer Convergence Protocol, or PLCP, header 20), to four clients. The clients also return their Acknowledgements 6, 7, 8, 9 after waiting a SIFS in the usual manner. However, in the upstream direction the AP first transmits a special trigger frame 22 (which has a PLCP header 21), which triggers a subset of clients to synchronize to each other and simultaneously transmit their PPDUs 26, 27, 28, 29 as a single OFDMA burst. The OFDMA frame has a PLCP header 23—transmitted by all clients simultaneously—as well as padding 24 to ensure that all clients transmit for an equal amount of time. The AP then returns a composite Acknowledgement 25 to all of the clients, indicating proper reception and acceptance of PPDUS 26, 27, 28, 29.
Trigger frame 22 plays several important roles in the OFDMA transmission. Firstly, it provides a common synchronization reference for all of the clients, which is required in order for the clients to align their OFDMA symbols with each other so that the AP can properly receive and decode them. Secondly, the trigger frame allows the AP to control which specific clients must transmit data upstream, thereby ensuring that the AP will be able to decode the client data when it is received. Finally, the trigger frame supplies critical parameters such as Transmit Opportunity (TXOP) duration, buffer state information, QoS state, etc. that the clients will need to know in order to control their OFDMA transmissions.
A possible example of trigger frame 22 is shown in FIG. 3. As represented, trigger frame 22 may comprise an IEEE 802.11 frame header 30, followed by a block of common data 31. This data may provide parameters such as TXOP duration and buffer state information that is required by all clients wishing to send upstream data. Following common data block 31, individual client information blocks 32, 33 may encode per-client data. The per-client data may indicate the specific client that is required to transmit upstream data at a specified time after the receipt of this trigger frame, as well as any client-specific parameters or control information that may be needed. Finally, a standard IEEE 802.11 frame trailer 34 may be located at the end of the trigger frame.
It will be apparent from FIG. 2 that a significant improvement in efficiency is obtained by the use of OFDMA in the upstream direction. Instead of each client having to transmit its PPDU individually, and also insert SIFS and DIFS spacings between the various PPDUs, it is possible for the clients to concurrently use the same radio channel to send all their PPDUs at the same time. Also, the AP can transmit a single Acknowledgement packet 25 concurrently acknowledging receipt of the four PPDUs to all four clients in one frame, further improving the efficiency and eliminating much of the delays due to the SIFS gaps.
However, the use of trigger frame 22 still has some significant limitations on efficiency. According to the IEEE 802.11 protocol, the AP must wait for at least a DIFS time before attempting to regain the medium after the clients return their Acknowledgement frames 6, 7, 8, 9. Further, the transmission of trigger frame 22 has its own overhead in terms of a PLOP header, and the transmission of the OFDMA frames by the clients cannot occur until at least a SIFS time after the trigger frame is received. When a large number of frame exchanges are occurring over a relatively short period, as would be common for high-bandwidth exchanges between the AP and its clients, these overheads can become substantial, resulting in a significant drop in efficiency and a reduction in usable channel capacity. The known prior art therefore suffers from serious shortcomings with regard to attaining the best efficiency for OFDMA operations. There is hence a need for improved wireless data communication test systems and methods. A system that is capable of reducing the overhead of trigger frames is desirable.